The present invention relates to a method and a device for symmetrically modulating a controlled series compensation system in electrical networks, particularly those including power supply systems.
In series compensation systems, capacitors are typically employed in the wiring run to reduce the current-dependent voltage drop across the line and the transmission angle, in steps. These capacitors are capacitor banks, which are switched on and off in series, as a whole or in several capacitor sections or segments. The capacitor is switched on and off by opening or closing a parallel circuit-breaker. In case of a short-circuit in the network, a parallel arrester, a triggerable spark gap, and/or a parallel circuit-breaker guarantee protection for the capacitor.
Furthermore, a series compensation system is known, in which by means of an inductor connected parallel to the capacitor, the total impedance of this series capacitor (similarly to the case of a thyristor controlled reactor or TCR in the static compensator) is steplessly controlled with a current-converter valve to high-voltage potential through appropriate triggering. A series compensation controlled in this manner is known as an ASC (Advanced Series Compensation). A series compensator controlled in this manner allows the dynamic response of the series compensation to be improved, and the total impedance may be controlled automatically within a certain range, whereby the impedance can be changed from capacitive to inductive.
These types of series capacitors are introduced in the essay, Automatically Controlled Parallel- and Series Compensation [Geregelte Parallel- und Reihenkompensation], published in the German periodical, "Elektrie", Volume 45, March 1991, pp. 88 through 90. In addition, the International Patent WO 87/04538 describes a series capacitor, which is controlled in such a manner and is integrated in a transmission line.
A controlled series compensation system always has a three-phase design in accordance with its application in series-compensated maximum voltage networks. Its principal tasks are: influencing conduction current (manipulating load flow), influencing bus voltage, and damping line circuit oscillations. These tasks can be fulfilled by a same kind of modulation of the three branches of the controlled series compensation system. The firing angles in these three branches are then the same. A loop controller that is common to the branches of the controlled series compensation system requires an actual value, which is determined in accordance with a procedure defined by the control task from three, possibly unequal measured values. This can comprise eliminating or limiting strongly deviating individual measured values, as can occur when asymmetrical network conditions prevail.
If the task also involves equalizing the conduction-current or conduction-voltage amplitudes (fundamental wave), then this can be solved by forming a separate control loop for each branch. However, this could cause the modulation in the branches to vary considerably (for example, in the case of asymmetrical network faults), which would entail the following disadvantages:
A different impedance can be adjusted for each line; consequently the degree of compensation is not the same in all three lines of one network.
The components of the controlled series compensation system, in particular the capacitor bank, are loaded to varying degrees; the most heavily loaded capacitor bank determines when the entire compensation system is blocked for reasons of protection.